Determining the class of a string

Although a string is generally considered to be an alphanumeric character, it can also belong to a class of strings. These classes allow us to manipulate the string in a manner consistent with its class type. For example, adding 1 to the character # will produce an error or unexpected return value. To assist us in determining the class of the string, Tcl provides the is keyword and a list of associated classes.

The syntax for the string command is as follows:

string is class strict failindex variable string

When invoked with the is keyword the command will return 1, if the class is matched.

The classes is will check for are as follows:

Class	Description
alnum	Any Unicode alphabetic or digit character.
alpha	Any Unicode alphabetic character.
ascii	Any character with a value less than \u0080 (those that are in the 7-bit ASCII range).
boolean	Any of the forms allowed by TCL_GetBoolean.
control	Any Unicode control character.
digit	Any...